1. Field of the Invention
The present invention relates to data storage subsystems employing a controller to store data on a various physical media while representing to host applications that data is always stored in sequential storage media such as portable magnetic tape cartridges. A router satisfies selected host commands without involving the controller. For instance, the router carries out host read/write commands by conducting data between physical data storage and the host, bypassing the controller and thereby reducing controller workload.
2. Description of the Related Art
A typical virtual tape subsystem (VTS) utilizes a VTS controller along with some disk and tape storage. The VTS controller manages physical data storage of data upon disk and tape, while representing to host applications that data is exclusively stored on portable items of media such as tape cartridges. The VTS subcomponents are configured such that all data moves through the controller. Namely, data to/from the host always passes through the controller. From the controller, data moves to/from disk. According to an appropriate destaging algorithm invisible to the hosts, the controller moves data from disk to tape. Since all data passes through the controller, the controller becomes a bottleneck for data movement.
Various outboard data copy techniques are known. Generally, these techniques utilize a separate processor or program to move data from source to destination, thereby relieving a targeted application from this workload. One example is the SCSI Extended Copy (XCOPY) command. The VTS controller may experience some relief by invoking XCOPY or another outboard data copy technique to transfer data from disk to tape when required. Despite this aid to the controller in regard to disk-tape or tape-disk transfers, the VTS controller is still an essential bottleneck for hosts' read/write requests. Namely, write data necessarily passes from the host to disk/tape through the VTS controller, and read data necessarily passes from disk/tape through the VTS controller to the host.
Routing all data through the VTS controller in this manner places a significant burden on the VTS controller, ultimately delaying completion of host read/write operations. From a host (customer) perspective, this is undesirable because it forces host applications to wait longer to read and write data. From a system design standpoint, the VTS controller bottleneck issue is undesirable because read/write speed is an important property that determines whether a VTS product will be competitive in the marketplace. It is essential to invoke the VTS controller, however, because the VTS controller oversees the arrangement of data on disk/tape, including storage addresses, maps between virtual and physical locations, and other metadata. Consequently, from the standpoint of read/write speed, known VTS type data storage systems still have certain unsolved limitations.